How fast can you spin a radiometer?

I'm curious how many of you have a radiometer, and if so, what's the fastest you've spun it using a flashlight. Subjective, I know.

Faster than the sun can propel it on a clear day?

I'd love to see what happens to one with the 500+ lumen flashlights, or even the HID spots.

A radiometer is that little lightbuld looking device that has the 4 diamond shaped flags on wires inside, that reflect light from one side and absorb it on the other. When you expose it to strong enough light, it will start spinning. The light actually pushes on the arms holding the flags, which causes the whole inner assembly to rotate.

I used to have a radiometer, but I never thought to try using a flashlight on one.

You just watch, this'll become the de facto standard for measuring light power.

I've got an RC airplane prop rpm measuring tool somewhere, this sounds like a fun experiment just for doing-sake.

For those not familiar with these interesting devices, here's a link:

http://en.wikipedia.org/wiki/Crookes_radiometer

I used to have two radiometers; one with two vanes and another with four vanes.
If I remember correctly, the one with two vanes became broken in an earthquake in early-2001, and I'm not 100% certain if I saved the other one when I moved in late-2004. I'd have to check a couple of boxes with glass telephone insulaters in them downstairs to see if it is inside one. I kept it in the window with the insulaters, so that would be the most logical place to check first.

(Edit, a few moments later)
I just went downstairs to check, and I could not find the boxes, so I can't check for my radiometer.

I do not own or have access to a noncontact tachometer anyway, so I would not be prepared to measure speed other than just by eyeballing it.

I'd like to see how fast I could get a radiometer spinning with one of these:
http://www.goldmine-elec-products.com/prodinfo.asp?number=G13994&variation=&aitem=2&mitem=3 :huh:

Since these rascals run on heat *difference* you can make 'em go faster by putting the radiometer in the freezer for a while first.

Another important point is it isn't a vacuum there, you need some (small number) of gas molecules to get heated up by the dark side, raise the local pressure, and push it around. Photons only bring the energy in. Least ways that's the way I recall it....

Doug Owen

A true vacuum is very difficult and very expensive to attain. So naturally a "vacuum" will have some air, although I'm not sure exactly how radiometers work.

I had a radiometer, I think I broke it by snapping the stem under the base, destroying the vaccuum. Never held my interest for more than a few minutes anyhow !!

I think any good flashaholic could devise a way to measure RPM's. For instance I had a large electric motor that I wanted to know the no load RPM pseed of. It had a hole driled through the side of the drive shaft so I taped 2 IR led's that I had and taped them to the motor body so that wheen the shaft was rotated one led would shine through the hole on to the second one. This fact that an LED will generate a very small voltage when light is shown on it is not a widely advertised fact and I would supose it would be most responsive to the wavelength of which it emits? Maby? Whell I hooked up the recieving LED to my multi meter and set it to frequency counting. You can figure the math. I would suspect that something similar could be made to work using a laser and an led even with an HID light shining on the radiometer?

carrot said:

A true vacuum is very difficult and very expensive to attain. So naturally a "vacuum" will have some air, although I'm not sure exactly how radiometers work.

Click to expand...

Agreed, there is no such thing as a perfect vacuum, even free space has a molecule or two of Hydrogen per cc. However, in this case we don't want a perfect vacuum, in fact we don't even want a really good one.....or even a medium one. The reference cited earlier in the thread spells it out:

"The effect begins to be seen at partial vacuum pressures of a few millimeters of mercury (torr) , reaches a peak at around 10-2 torr and has disappeared by the time the vacuum reaches 10-6 torr. [see note 1]. "

This is mechanical pump country, not even Diffusion or Turbomolecular pumped. Let alone cryo or ion pumped. Several orders of magnitude (like 1000 times) 'worse' than a common vacuum tube or CRT for instance.

Nothing fancy needed.

The electron microscopes I work on are hundreds of times better than this range for routine machines, high vac ones thousands better, UHV (Ultra High Vacuum) ones millions of times better....and we do that every day.

Doug Owen

Brlux said:

...Well I hooked up the recieving LED to my multi meter and set it to frequency counting. You can figure the math...

Click to expand...

I thought about doing something like this already, but none of my DMMs has a frequency counter function. :shakehead:
So the idea went nowhere fast.

Not all is lost LED, I think you could use that setup for a tach.

Ive seen those things, but never actually played with one. Ive seen quite a few items in the edmunds scientific catalog I would like to own and play with.

Ive never understood it, but Tesla and Maxwell had lots of info in their writings about how to draw free energy from a vacuum. Maybe they were just referring to the lack of wind resistance?

When I had to measure RPM of a motor, I paint a spot with reflective paint on the axis. I use a photodiode and an o'scope (or a frequency counter) to read the number of pulses. No need to drill holes.
Another possibility is the stobo meter. With the same reflective spot on the axis, use a pulsed light to illuminate it. When the spot seems to stop, the rotating speed is equal to the number of pulses of light, or multiple of it.

Anthony

The_LED_Museum said:

I thought about doing something like this already, but none of my DMMs has a frequency counter function. :shakehead:
So the idea went nowhere fast.

Click to expand...

So how about instead of the DMM you don't have, you feed it to the microphone input on the computer sound card you do have? You can record a short audio file of the result and either read the period with an audio eidt program (I've got a copy of a free program called 'cooledit' or something like that that does this), or email the file to someone who can....

I suspect you can 'read' the signal with a simple photo transistor 'watching the vanes flash past'.

Doug Owen